A senior police officer has become the first person to be convicted as a result of the £40m investigations into phone hacking and corruption of public officials.

Detective Chief Inspector April Casburn, 53, was found guilty of misconduct in public office at Southwark crown court after the jury decided she had tried to sell information from the phone-hacking inquiry, which was set up in 2010, to the News of the World.

Mr Justice Fulford warned Casburn, a mother of three, that she faced an immediate custodial sentence and the Metropolitan police said she had "betrayed the service and let down her colleagues". But Patrick Gibbs QC, her counsel, asked the judge to take into account the fact that Casburn was in the process of adopting a child. Sources close to her said she was reeling after being told she could face a five-year jail term because the judge wanted to make an example of her.

The offence took place as the Metropolitan police were forced to re-examine allegations of phone hacking at the News of the World after revelations in the Guardian and the New York Times that the activity was widespread and not just relegated to "one rogue reporter" as News International, the publisher of the now defunct tabloid, had maintained for years.

It spawned three linked investigations – Operations Weeting, Elveden and Tuleta – into phone and computer hacking and the corruption of public officials. Casburn is the first to be convicted in any of them.

Fourteen individuals, including the former NI chief executive Rebekah Brooks and Andy Coulson, the ex-NoW editor and former No 10 director of communications, face trial later this year for offences including perverting the course of justice and conspiracy to illegally intercept phone messages. The charges include allegations that some of the 14 were involved in hacking into the mobile phone of the murdered schoolgirl Milly Dowler.

The jury in the Casburn trial believed the crown's case that she had asked for money and sought to give the newspaper at the centre of the investigation information. Det Chief Supt Gordon Briggs, who oversees Operations Weeting, Elveden and Tuleta, said: "It is a great disappointment that a detective chief inspector in the counterterrorism command should have abused her position in this way.

"There's no place for corrupt officers or staff in the Metropolitan police service. We hope that the prosecution demonstrates that leaking or in this case trying to sell confidential information to journalists for personal gain will not be tolerated.

"There may be occasions when putting certain information into the public domain, so called whistleblowing, can be justified. This was not one of them. In this case DCI Casburn approached the News of the World, the very newspaper being investigated, to make money."

Casburn, who was manager of the national financial investigation unit with counterterrorism, claimed she had never asked for money when she telephoned the News of the World on 11 September 2010. She said she made the call because she was concerned that resources from counter-terrorism and her unit were to be diverted into a new phone-hacking investigation.

She had attended a meeting with colleagues the day before in which it was revealed that John Yates, the Met's then assistant commissioner, was to reopen the phone-hacking investigation. She said her colleagues were joking about the inquiry, and were excited about who would get to interview Sienna Miller.

"I felt very strongly that we shouldn't be doing hacking. Our function was to prevent terrorist attacks and I was particularly worried that the behaviour of my colleagues was such that they thought it was a bit of a jolly. It made me really angry."

She said she regretted making the telephone call but defended ringing a newspaper because she said she was not an influential member of the counterterrorism team and would not have been listened to if she raised concerns. She said she had been bullied for two years. "I think in some circumstances it is right to go to the press, because they do expose wrongdoing and they expose poor decisions," she said.

But the jury rejected her defence, accepting the crown's case that she had phoned the NoW to say that Coulson and five other people were being investigated by the Met and had asked for payment for the information.

Mark Bryant-Heron, prosecuting, said she was tipping off the paper and offering to sell information. "This was a gross breach of the trust the public had in a senior police officer."

Casburn was arrested 15 months after she made the call as a result of a huge tranche of evidence, including 300m emails, handed to the police by the NI management and standards committee.

The reporter on the News of the World who took the call, Tim Wood, wrote an email to more senior colleagues, detailing what he claimed had been said. It was the crown's main evidence against Casburn.

It read: "PHONE TAPPING. A senior policewoman … who claims to be working on the phone-tapping investigation wants to sell inside info on the police inquiry. She says the investigation was launched yesterday (Fri) by Yates and he is using 'counter-terrorist assets', which is highly unusual. An intelligence development team is being used and they are looking at six people. Coulson, Hoare and a woman she cannot remember the name of. The three other people used to work for the News of the World and police do not know where they are now (she did not know their names either). Pressure to conduct the inquiry is coming from Lord Prescott."

Casburn will be sentenced later. Her barrister said he would be seeking a suspended sentence. She is of previous good character and has a flawless disciplinary record. The mother of two adult children, who recently adopted a young child with her third husband, she left school after O-levels and joined the Met in 1993, serving in the child protection unit before moving on to counterterrorism. She is suspended and faces the sack. She detailed to the jury how she had suffered two years of bullying within the counter-terrorism unit, and as the only woman within her department who had not been given a desk.